A source in an oil and/or gas production system may for instance be a well, a production zone, a reservoir zone or a reservoir. The flow from each source is typically a multiphase flow comprising a composition of oil and gas related components, brines, metals, water, etc. The flow may be defined as mass rate or volume rate. Typically, the pipelines from a plurality of sources are by some means connected to a single pipeline. In the case of offshore oil and gas production this may be a riser, which brings the total flow to a production facility above sea level. An example of such a production facility is a production platform, a production ship or an on-shore production facility. This means that flows from different sources have been blended as they go into the production facility. Hence, the allocation of flow rates to each source is a major challenge.
Prior art shows that direct measurements of flow rates in multiphase flows are possible by using multiphase flow meters. As already mentioned, the flows from several sources are often blended in a single riser, so to measure the rates from each source directly, one would as an example need a multiphase flow meter associated with each of them.
There exists prior art in the field of analysis of oil and gas composition for a multiphase fluid. U.S. Pat. No. 5,166,747 describes an apparatus for analyzing composition of formation fluids in a borehole. The apparatus distinguishes between formation fluids and mud filtrate. The apparatus use absorption spectral information to determine the composition of the fluid.
EP 859236 shows a method for controlling a process for separating at least one component from crude oil, which flows into a pipeline and then through a separator. The method comprises the step of analyzing a sample of crude oil before it enters the pipeline to give an analytical result before the crude oil, from which the sample was taken, reaches the separator.
WO 9414060 describes a composition analyzer for determining the composition and flow rate of a multi component, multiphase fluid containing a gas. The analyzer comprises a conduit for receiving the flow of the multi component and multiphase fluid. A section of the conduit is heated. The analyzer comprises means for measuring the temperature, pressure and flow rate of the fluid at points closely adjacent to, and upstream and down stream from the heated section of the conduit. Measurements are sent to a programmable logic computer, the computer being programmed to use thermodynamic and gas law relationships to calculate the fluid composition and the flow rates of the fluid compositions.
One approach to allocate flow rates to sources is to apply computer simulations to determine the flow rates that give the best possible fit to measured flow properties such as pressure and temperature. Such measurements may be available for each source or the pipeline transporting fluid from the source. A remaining problem is that the success of such an approach depends on the existence of the previously mentioned measurements and further it depends on whether models and computer simulations are reliable. A problem is that the mentioned measurements are, due to cost reasons, often not available. Another problem is that, if such measurement equipment does exist but fails it is expensive to repair since it may be situated sub-sea and even down-hole. Even if such equipment is repaired or substituted, a remaining problem is to estimate or calculate flow from the sources during times when measurements are unavailable.
It is known that fluids produced from different sources typically have slightly different compositions. A mixture of a certain oil, gas, and water is an example of a fluid. The fluids originating from different production sources consist mostly of the same components, but usually the concentrations vary slightly from one source to the other. The paper “Characterizing the Greater Burgan Field: Use of Geochemistry and Oil Fingerprinting”; SPE 37803, published by Society of Petroleum Engineers, Inc. 1997, describes that oil fingerprints are identified for producing reservoirs and are used to identify wells with mixed production because of wellbore mechanical problems.
Another problem is that there is an increased demand for improved reliability and robustness of flow allocation performed in existing installations where on-line measurements of flows from individual sources are available. For instance, there is a need to continue to perform flow allocation, when such on-line measurements are unavailable due to equipment or communication failure.